Electric motors have a stator and a rotor which is rotatable with respect to the stator. In an outer rotor motor the rotor is disposed around the stator. In a brushless DC motor the rotor has magnetic poles formed or created by one or more permanent magnets, while the stator has windings which form the magnetic poles of the stator. The stator core is a key component for mounting and positioning of the stator windings and providing magnetic flux path for the stator. The stator core is usually formed by a stack of core laminations of silicon steel.
Because the stator core has a magnetic resistance to the magnetic flux, during operation of the motor, the magnetic flux passing through the stator core generates heat. In addition, because the stator core itself is a conductor, during operation, an induction potential is generated in a plane perpendicular to the magnetic lines. This induction potential produces an eddy current in a transverse section of the stator core, such that the stator core generates heat thus resulting in an eddy current loss of the motor. The hysteresis loss and eddy current loss both occur in the stator core and are collectively referred to as iron loss of the motor. The use of laminations reduces the eddy current loss but they can still be significant.
As shown in FIG. 1, a core lamination 100 for forming the traditional stator core includes an annular portion 130, a plurality of tooth portions 150 extending radially outwardly from the annular portion 130, and a tip 170 disposed at a distal end of each tooth portion 150. Each tooth portion 150 is elongated with a large height, i.e. a large distance from a root (where the tooth portion 150 connects to the annular portion 130) of the tooth portion to the tip 170. This results in a long magnetic path and hence a high magnetic resistance. The magnetic flux passing through the stator core formed by the core laminations 100 generates a large amount of heat, thus causing a significant iron loss and hence a poor efficiency of the motor.
In order to enhance the motor efficiency, it is necessary to improve the stator core to reduce the iron losses. A typical method for reducing the iron loss is the use of low iron loss silicon steel. However, low iron loss silicon steel has a higher cost, which translates into an increased overall cost of the motor.